Silicon carbide has been formed heretofore by pyrolysis of polycarbosilane polymers. See for example U.S. Pat. Nos. 4,052,430, 4,100,233, and 4,159,259 to Yajima et al. However, as the patentees point out in their subsequent U.S. Pat. Nos. 4,220,600 and 4,283,376, there are a number of defects in such earlier processes. They indicate, for example, that for satisfactory results the polymer must be formed in either a pressure reactor or a circulating-type apparatus which permits recycling. In the case of the pressure reactor they note that the polymerization must be carried out at a temperature of 400.degree. to 470.degree. C. and a pressure of 80 to 110 atmospheres for 10 to 15 hours, that the provision of pressure-resistant equipment and measures against the danger of fire are essential, and that the method is not suitable for mass production.
They point out that in the case of the circulating-type apparatus the equipment is extensive, recycle of lower molecular weight products is necessary, the temperature must be raised to as high as 600.degree. to 800.degree. C., and the reaction time must be as long as 20 to 50 hours.
In describing the defects associated with the process described in their U.S. Pat. No. 4,100,233, Yajima et al. point out that it is essential to perform the step of removing low molecular weight compounds from the polycarbosilane polymer before spinning the polymer into fibers, that the resultant polycarbosilane fibers obtained in the process are so brittle that they are difficiult to handle, that the tensile properties of these fibers are poor, and that breakage of these fibers tends to occur in the final firing step.
Consequently, after making extensive investigations to remove the defects of the prior art, Yajima et al. resorted in U.S. Pat. Nos. 4,220,600 and 4,283,376 to the inclusion of siloxane bonds in the polycarbosiloxane polymer by including a phenyl-substituted polyborosiloxane in the polymerization mixture.
Still other drawbacks exist in conventional processes for producing silicon carbide by pyrolysis of polycarbosilane polymers. For instance, low yields of the silicon carbide are often experienced. And the product often has an undesirably high impurity content. In fact, it appears that the highest reported yield based on thermal gravimetric analysis is 60%, and this material has been shown to have excess carbon as well as SiO.sub.2 impurities.